Radio receiver



Aug- 19, 1952 w. s. PERclvAL 2,607,889

RAD-[O RECEIVER Filed DSC. 20, 1946 ATTORNEY Patented Aug. 19,v 19522,607,889 e RADIO micrzivlm J William. S..WPercivlal, London, England,assignor L itoxElectric `a Musical Industries Limited, dlesex, England,a company of 'Great Britain Appucatiionnecembr zo, 1946, sriaN. 717,427f In Great Britain June 23, 194.5

This invention relates to receivers-.for amplitude` modulated radiofrequency'c'arrier Waves, and hassfor its object the provisionofimproved means for reducing or eliminatingV the veffects ofinterference. Theinve'ntion is based on vvthe fact that,' in general,`the i-nterference will cause phase modulation: of the carrier Wave.

According to the invention there is provided a receiver for amplitudemodulated carrier Waves, l

having a discriminatorcircuit for 4effecting automaticA frequency.control of Ithe receiver and wherein means 'are provided'wherebyan'output of: said discriminator circuitv in .response to phasemodulation of a received' carrier Wave is arranged to 'control the gainof said receiver'in such manner that said gain is'freduced'whenthecarrier wave is phase modulated or 'phase 'modulated to a predetermineddegree. n

In a preferred'embodiment of the' invention the output from said phasediscriminator circuit in response to phase modulation ,ofthe carrierWave is applied to an amplifying valvethe. output from which is appliedto the control of the gain of said receiver and wherein the automaticfrequency controllvoltage of said discriminator circuits-is also.applied to said amplifying valve so as tobias said valve to cut-off whensaid receiverifsibeing tunedlorwh'en the automaticfrequency controlvoltage. is operating to: correct the tuningy .of the receiver. Y i: l,

. In .order-'that the said invention may be-clearly understood and..readily carried =into effect', the samevvillnow be more fully describedwith reference .to .thefaccompanying drawing, the single figure' ofwhich is a circuit diagram of'partof a superheterodyne receiverembodying the invention.

Referring to the drawing the receiver has a conventional h-f amplifyingstage which includes h-f amplifying valve I the output from which is fedto the input circuit of an oscillator mixer valve 2 the local oscillatorcircuit of which includes an inductance 3 and a condenser 4, the

capa-citance of which is variable for the purpose of varying thefrequency of the local oscillations and so tuning the receiver. Theoutput from valve 2, consisting of a. modulated carrier wave ofintermediate frequency, is fed to the control electrode of an i-famplifying valve 5. The output circuit of valve 5 includes a coil 6which is inductively coupled with a coil 1 included in the input circuitof a diode triode detector valve 8, and the output from valve 8 is fedto an audio frequency amplifying stage (not shown). The coil I is alsoinductively coupled with the coil 9 5 claims. (Cl. 25o-zo) of a phasediscriminator comprising ax'double diode I0, the outputl circuit ofwhich comprises resistances II and I2 shunted by agcondenser YI3'. Theoperation of a frequency discriminator of the kind illustrated is Well4known and it'willsulce to say that if the mid-frequency of the i-fcarrier wave in the output of valve 5 isthe same as the predeterminedmean frequency for which the discriminator is adjusted, -equal 1 andvopposite potentials appear across the resistances 'I I and I2.Ifrhowever the frequency of the l-fxcarrier Wave-departs from this meanfrequency, by rea'- son of the receiver beingoff-tune, thepotentialsacross resistance Il and I2 will have a resultant the signv of whichdepends on the direction in which the i-f carrier frequency departs fromthe said mean frequency. This resultant potential is applied through thelow passsmoothingfllter II, I3, I4', I5 of relatively long time constantto the controlelectrode of .valve 5, andthe'resulting change -in 'thelrelative potential of this `control electrode causes an alteration inthe current flow throughvalve 5 with the result'that the degree ofsaturation ofthe core of the transformerzc'om'- prising inductance I6 inthe cathode leadofg-valve 5 and the inductance` I'Iisrvaried, therebyvarying jthe ,effective inductance' of the inductance Il and likewise ofthe. coupled inductances `I8 and 3. The tuning ofthe oscillatorv.circuit :of valve 21s accordingly altered Whereby'the frequency of.the local oscillations is altered -so that the i-f again becomes equaltothe predetermined meanrfref quency of the discriminator. x

The output circuit'of the double diode rectifier IIJ is also connected,through a condenser I9, to the input circuit of an amplifying valve 2U.In the event of the i-f carrier Wave in the outputl from valve 5 beingphase modulated as a. result,

of interference, such modulations will be detected by the discriminatorand converted into audio frequency potentials across the resistances II, I2.

The time constant of the resistances II and I2- and the condenser I3 issuiciently short to pass audio frequencies. These audio frequencypotentials are applied to the control electrode ol' amplifying valve 20the output circuit of which includes a transformer 2l feeding a fullwave rectifier 22, the output from which is applied,

after smoothing by filter 23, 24 of relatively short time constant, forexample, 0.01 second, to the controlelectrode of valve I, saidrectification ensuring that the potential applied to the controlelectrode of valve I is always of such sign as to tend to bias valve Ito cut-off. The normal bias applied to valve 20 is preferably such thatthe I is not cut-off until the interference If desired a long valvereaches a predetermined level.

y time constantv circuit may be included so as to prevent audiofrequency potentials from reaching the control electrode of valve 5.

Phase modulation of the i-f carrier may result when vthe receiver isbeingtu'ned. andto prevent the valve I from being cut-'off during tuninga switch 25 is provided in the connection from the discriminator to thecontrol elec'- trode of valve 20. This switch 25 should be opened duringthe operation of tuning ofthe re' carrier waves.

ceiver but closed when the receiver is'in tune;

and transient interference is experienced. `The switch 25 may be coupledto the tuning control of the receiver so that it is automaticallyopened.

2. In a radio receiver for amplitude modulated :carrier-waves, thecombination as defined in claim "1, in which the first filter meanscomprises a rewhen the tuning control is being actuated. Al-

ternatively, the frequency control voltage smoothed by the filter I4, I5may also be applied to the control electrode of valve 20, in the samemanner as to valve 5, in addition tothe audio frequency voltage from thediscriminator, in such manner, as by elimination of the condenser I9,that provided the valve 20 has properv grid-bias anode-currentcharacteristic, it is biased olf by said frequency control Voltage whenthe receiver is off-tune and is permitted to operate under the control'of the said audio frequency voltage only when the receiver is in vtuneand no frequenc control voltage is present. i

It will be understood that the audiofrequency control voltage may beapplied to any suitable valveof the receiver for the purpose of reducingthe receiver gain during periods of interferenceuthat is, it neednotlnecessarily be applied,

`as-shovvn, to the h-f amplifying valve I.

`What I claim is:

11..,1n a radio receiver foramplitude modulated carrier waves, thecombination with a radio frequency amplifying stage, a local oscillator,and .afrequency converting stage coupled to said amplifying vstage andto said oscillator, of an intermediate :frequency carrier waveamplifying stage having input and output circuits, said frequencyconverting stage being coupled to said input cir- K cuit,dis'criminatingmeans coupled to said out- 'put circuit and havingia load circuit inwhich .output potentials vary lin accordance with fre-- quencyvariations of the input circuit, a first lterrmeans coupling said loadcircuit to said age being developed across said rload circuit insistance-capacitance lter having a relatively long 4time constant.

erative during tuning of; said radio frequency amplifier stage. -v

5. In-aradio receiver for amplitude `modulated carrier waves, the.combination as defined in claim ,1, in which there is provided a firsttuning means for the radio frequency amplifying stage, a .second tuningmeans .for the local oscillator, and unicontrol means linking saidtuning means and operative during tuning operations to render therectifying means inoperative.

WILLIAM SPENCER PERCIVAL.

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